This invention relates generally to gas furnaces and, more particularly, to a method and apparatus for detecting a fault condition in a gas valve relay and for responsively closing the gas valve by shutting off the power thereto.
In the operation of gas fired furnaces, it is imperative that the flow of gas be closely controlled so as to be coordinated with the other related functions that support the combustion process. That is, to ensure the safe and efficient operation of an induced draft furnace, it is important to precisely control the sequence and timing of each step of the heating cycle wherein, typically, the inducer motor is turned on to purge the system, the ignitor is turned on and allowed to warm up, the gas valve is turned on to initiate ignition, a sensor is caused to function to ensure that a flame does exist, and finally, the circulating air blower is turned on to provide the flow of hot air to the space to be heated. When the set temperature of the thermostat has been satisfied, the gas valve and inducer motor are turned off and then, after an appropriate delay period, the circulating air blower is turned off.
The gas valve in such a system is typically opened and closed by way of an electromechanical relay which selectively turns on or turns off power to the gas valve by way of the closing or opening of power contacts by way of the control circuit. Generally, the relay contacts are designed so that if the relay is operated within its specified operating conditions, the relay will provide reliable operation without the occurrence of failure. However, in the event that abnormal operating conditions should occur, such as, for example, a power line braking in an ice storm to thereby cause power surges to travel through the line to the control, a welding of the relay contact to the closed position could occur. This, in turn, would cause the gas valve to remain open when it should later be closed. For example, if the system is operating with the power contacts closed and the gas valve on during the occurrence of such a power surge, and as a result the contacts are welded closed, then at the end of the heating cycle the control system will turn off the inducer and will try to turn off the gas by operation of the relay. But since the contacts are welded close, the gas valve will remain open. Without combustion air being supplied by the inducer, the combustion process will be inhibited and a buildup of gas will result. This may in turn cause an undesirable flame roll out with possible resulting damage to the furnace.
A common technique for protecting against such a fault is to provide a fuse in series with the power contacts. When a relay contact failure has been detected, an auxiliary circuit is turned on to thereby draw sufficient current to burn out the fuse, thus opening the electrical circuit to cut off power to the gas valve. The existence of a fuse in the circuit, however, can become a nuisance. For example, if the fuse fails due to fatigue, as they frequently do, or if a short circuit is accidentally caused during servicing of the unit, the fuse will be caused to burn out, and the system will be inoperable until the burned out fuse is detected and replaced, a process which may be inconvenient and costly to the owner.
It is therefore an object of the present invention to provide an improved control system for a gas fired furnace.
Another object of the present invention is the provision in a gas fired furnace for safely and reliably dealing with the occurrence of a gas valve relay fault.
Yet another object of the present invention is the provision in a gas fired furnace for eliminating the use of a fuse in the power line to the gas valve.
Still another object of the present invention is the provision in a gas fired furnace for dealing with the problem of welded relay contacts without the use of a fuse and associated auxiliary circuitry.
Yet another object of the present invention is the provision in a gas valve control system which is economical to manufacture and effective and safe in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.